With regard to x-ray systems, a transition to digital imaging methods has been taking place for some time which has resulted in solid-state detectors being developed for digital x-ray imaging which are based on active readout matrices. The detectors are made of amorphous silicon (a-Si), the image information is converted in an x-ray converter, which is made of cesium iodide (CsI) for example, and converted into electrical charge and stored in photodiodes of the matrix. Similar technologies likewise use an active readout matrix made of amorphous silicon but employ a converter which generates electrical charge directly that is then stored on an electrode. The stored charge is subsequently read out by way of an active switching element with dedicated electronics and converted from analog to digital and processed further by an image system.
All semiconductor detectors, including digital solid-state images detectors in particular, require that the calibration data be regularly updated during operation. In order to achieve optimum quality of the x-ray images, so-called offset images are acquired without using x-ray radiation for the acquisition. After the exposure of the detector the image information is read out, followed by a reset phase. As a result of the acquisition of the calibration data in the form of offset images it is possible to compensate for interference effects which are caused by changes in temperature and other effects. Although a complete cycle for generating an offset image lasts only a few seconds, the acquisition of the calibration data by the x-ray system can only be initiated when the system is not active, in other words when no patient is present.
In practical applications the acquisition of the calibration data is normally performed automatically. With regard to conventional x-ray systems, an offset image is produced when no x-ray radiation has been applied during a predefined period of time. In this case it is assumed that there is no longer any patient in the examinant ion room and also that no x-ray radiation will be emitted in the immediate future which would prevent the acquisition of calibration data, for example as a result of the acquisition of offset images. This procedure results in relatively long time intervals being defined for the acquisition of the calibration data, with the result that the calibration data is not acquired in close temporal proximity. However, this solution is less than optimal in respect of the temperature dependency of detectors based on semiconductors but also in the case of detectors based on amorphous silicon.
DE 103 32 834 A1 describes a method for defect detection in the case of a digital x-ray detector and also an associated x-ray unit, in which are specified a method for the automatic detection of defective sensor elements of an x-ray detector and also an x-ray unit suitable for implementing the method. In this situation, an x-ray detector is provided which comprises a readout matrix applied to a substrate and which is provided with an illumination unit. The readout matrix is irradiated by means of the illumination unit with light of a predefined light intensity and a calibration image is acquired in the thus illuminated state. Using the calibration image, defective sensor elements are identified by means of a calibration unit, the number of defective sensor elements is ascertained and a warning message is issued when this number exceeds a predefined limit value.